1. Field of Invention
The present invention relates to technology of thin film transistor (TFT). More particularly, the present invention relates to a structure of TFT and a method for forming a TFT using a crystallization property from a excimer laser.
2. Description of Related Art
In general for the specification, the semiconductor material includes, for example, silicon or germanium, and the transistor is fabricated by the semiconductor material and can be used to form a device, such as a displaying device, a switching device and so on.
In recent years, a lot of research and development are made about the TFT, which is commonly used in the driving circuit for the active liquid crystal display (LCD) or the active organic electro-luminance (EL) display. The TFT can be generally categorized into a polysilicon TFT (poly-Si TFT) and amorphous-silicon TFT (a-Si TFT).
For the poly-Si TFT, it has rather higher mobility, so that in the future it can be used to form the pixel switching device or the peripheral driving circuit for the displaying device but also it can be used in a memory device or an information processing system.
If the information processing circuit is desired to be fabricated by using TFT, the mobility of TFT should be greater than 500 cm2/V-s. However, since the current polysilicon grain boundary usually is the scattering center with respect to the electrons, the mobility is hard to be greater than 100 cm2/V-s. In order to achieve 500 cm2/V-s for the TFT, it is necessary to use the single-crystal silicon TFT (c-Si TFT).
The conventional method to form a polysilicon film under a temperature of 500° C. is using an excimer laser to irradiate the a-Si film, so as to cause the silicon film to be melt and re-crystallized. Since this method does not produce a thermal damage on the substrate, it is considered to be highly effective. However, this method can only obtain the polysilicon film with relatively smaller grain size.
Recently, a sequential lateral solidification (SLS) was proposed by J. S. Im et al. (Phys. Stat. Sol. (a) 166(1998)603), which successfully produce a polysilicon film with the grain length of 100 microns. Since this grain size is longer than the channel length of the TFT, the electrons in motion actually do not cross over the grain boundary. As a result, this is equivalent to achieve the c-Si TFT, of which the mobility can be greater than 500 cm2/V-s. However, the method has several disadvantages that it needs a more expensive optical system, the using efficiency for the laser source is low, and the property of TFT is lack of uniformity due to a poor control of the grain boundary along the direction parallel to the scanning direction.
In addition in recent years, a method is also developed, wherein the crystal grain is formed at the determined position, so as to reduce the grain boundary at the TFT channel region, such as changing the grade level of the light intensity for the laser. Again, this method is also relying upon the expensive optical system, and the grain boundary along the lateral growth derection cannot be controled.